PATTERNS  OF  DISCHARGE  AND  SUSPENDED  SEDIMENT  TRANSPORT  IN  THE  WALNUT AND  SQUAW  CREEK  WATERSHEDS,  JASPER  COUNTY,  IOWA:  WATER  YEARS  1996-1998

Keith E. Schilling

Iowa Department of Natural Resources, Geological Survey Bureau
Technical Information Series 42, 2000, 47 p.

ABSTRACT

Sediment is the major pollutant affecting Iowa’s streams. Despite the magnitude of sediment impacts on streams, little research has been conducted to determine the nature of sediment transport in small Iowa watersheds. In the Walnut Creek and Squaw Creek watersheds in Jasper County, Iowa, research as part of the Walnut Creek Nonpoint Source Monitoring Project is focused on monitoring daily discharge and suspended sediment in paired, agricultural watersheds.  The Walnut Creek project was established in 1995 as a nonpoint source monitoring program in relation to watershed habitat restoration and agricultural management changes implemented by the U.S. Fish and Wildlife Service (USFWS) at the Neal Smith National Wildlife Refuge. The monitoring program utilizes a paired-watershed approach, with Walnut Creek as the treatment watershed and neighboring Squaw Creek as the control. Standard U.S. Geological Survey gaging facilities are located at upstream and downstream locations on Walnut Creek and downstream on Squaw Creek.  The purpose of this report is to present the results of three years of daily discharge and suspended sediment monitoring in Walnut and Squaw Creek watersheds and to examine the timing, frequency and magnitude of discharge and suspended sediment transport. 

Daily discharge and suspended sediment transport in both watersheds was very flashy, responding rapidly to precipitation and snowmelt events.  This pattern is typical of incised channels where flood events are contained within the channel and rapidly transport suspended sediment downstream. Five days in any given year accounted for 60 to 80% of the annual sediment load in both watersheds. Discharge and suspended sediment loads are highest in May and June, which accounted for 40 to 50% of the annual discharge and approximately 60% of the annual suspended sediment. The February to July period accounted for 98% of the annual sediment total.  Maximum peak flows and sediment loads and a higher proportion of annual sediment loads migrating during one- and five-day periods were observed in Squaw Creek than Walnut Creek.  Watershed morphology and land-use differences, including prairie restoration in Walnut Creek, may contribute to these differences.

A major source of sediment is streambank erosion of Holocene alluvium and post-settlement materials (up to 50% of the annual total). Streambed downcutting contributes little to sediment loads due to base level control provided by resistant Pre-Illinoian till. Other sources of sediment include contributions from upland sheet and rill erosion and concentrated flow erosion in gullies and tributary channels. Eroded sediment stored within the channel may take many years to exit the watershed.  A long-term monitoring record is needed to detect changes in the Walnut Creek watershed resulting from the land restoration efforts.  The long timeframe is necessary for factoring out influences of climate, variability in morphology and land use between Walnut Creek and Squaw Creek and the effects of historical sediment storage.  The Walnut Creek project may be capable of detecting changes more quickly than other projects due to the type and magnitude of land-use change being implemented, the presence of fewer tiles in the watershed and the dual gages located on the channel.

At a minimum, suspended sediment sampling should be conducted daily during the period February through July to characterize the flashy behavior observed in sediment loading during this time. In the August through January period, reduced sampling frequency, or estimating loads from daily discharge or turbidity can be used to estimate monthly sediment loads.